There are several methods disclosed in the literature for alloying high melting temperature metals such as manganese, chromium, and iron into aluminum. One method, disclosed in U.S. Pat. No. 2,911,297, involves the preparation of a briquette of powdered alloying metal and a dispersing agent. When such a briquette is introduced into a metal melt, the dispersing agent gives off a reactive gas or vapor, spontaneously disrupting the briquette and dispersing the powdered metal into the melt. This method has several disadvantages, including the undesirable "boiling up" effect on the molten bath caused by the evolution of gas from the dispersing agent which may result in undissolved powdered metal being floated to the top of the bath. Additionally, it is necessary to include a large amount (usually more than ten percent) of dispersing agent in the briquette.
A more recent method involves pre-mixing finely divided manganese containing particles with finely divided aluminum particles and compacting the mixture into a pellet or briquette. Brown et al. (U.S. Pat. No. 3,592,637) disclose that a briquette or compact comprised of a blended mixture of at least two different finely divided metal bearing materials rapidly dissolves when the briquette contains a "promoter material" such as finely divided aluminum. When such a compacted mixture is added to a bath of molten aluminum the lower melting portion of the mixture (i.e. the finely divided aluminum) melts and thus assists in the dispersion of the remaining mass of manganese. The speed at which the manganese then dissolves in the molten bath is merely a function of particle size, surface area, and bath variables such as temperature and amount of stirring.
Although offering improvements over previous methods, the practice described by Brown et al. also has several disadvantages. The finely divided aluminum needed to produce the briquette is expensive to purchase or produce. In actual practice, the composition of the briquette produced generally contains a significant proportion of aluminum, thus resulting in large quantities of briquettes being required to achieve the desired manganese content in the final alloy. Additionally, the alloying process described in U.S. Pat. No. 3,592,637 was carried out at bath temperatures of about 1400.degree.F. (760+C.) or higher. In actual practice, the temperature of the molten bath to which such a briquette is added is critical with respect to the rate at which the briquette will dissolve. At bath temperatures of about 1,280.degree.F. (693.degree.C.) to about 1,375.degree.F. (746.degree.C.), typical operating temperatures for aluminum baths in commercial operations, the solubility rate of such a briquette is markedly reduced. Thus this method suffers from the further disadvantage of requiring higher than normal operating bath temperatures to successfully incorporate a metal briquette into the bath.
Another recent method is described in U.S. Pat. No. 3,591,369. This patent discloses that manganese metal added to molten aluminum in the form of a manganese body having thereon a coating containing a complex potassium fluoride compound dissolves at a more rapid rate than a similar body of manganese without the coating. Theoretically the coating enhances the wetting of the manganese surface by the molten aluminum. This method is also desirable due to the costly process and controls required to produce coated manganese containing metal. In actual practice, this type of additive rarely yields recoveries above 90% and often takes in excess of one hour to achieve this value. Such incomplete recovery can result in a "build-up" of manganese in the furnace or vessel in which the alloy is being produced, causing serious metallurgical problems. In addition, this product requires bath temperatures of about 1375.degree.-1400.degree.F. (746.degree.-760.degree.C.) to obtain maximum solubility of the manganese in aluminum.
U.S. Pat. No. 3,793,007 describes a method for the addition of manganese particles having a specified size range (predominantly larger than 0.15 mm., or 100 mesh) to molten aluminum. The manganese is added in admixture with 3-10% of a fluxing agent which forms a molten phase at the temperature of the aluminum bath. The patent indicates that the alloying composition requires a substantial amount of fluxing agent as the particle size of the manganese is reduced, i.e. as the particle size range decreases toward 0.15 mm. the amount of flux required increases toward the 10% level. Some form of protective packaging to prevent undesirable absorption of moisture from the atmosphere by the fluxing agent prior to addition to the aluminum bath is also recommended.
Simply making a compact from finely divided manganese or other alloying constituents for addition to a molten aluminum bath is not a satisfactory solution. It has been found that if particulate alloying metal is to alloy into molten aluminum it is first necessary for the aluminum to contact and surround the particles. In the absence of some fluxing agent or "promoter materials", air remains trapped within the voids between the particles and prevents aluminum from pentrating such a compact, consequently preventing the required contact for complete solution. Nor is adding the finely divided alloying material to the molten bath in a unbriquetted or bulk form a practical solution. In bulk form, the fine particles tend to become trapped in the oxide or dross layer that exists in commercial practice on the surface of molten aluminum baths. The finer the particles, the more pronounced this problem becomes. This results in highly variable and usually poor recovery of alloying constituent in the final alloy.
Additionally, it has been a requirement of the newer types of alloying additives and processes for use in aluminum that the additive be introduced directly into a molten aluminum bath, rather than charged to an essentially empty furnace at the same time the other ingredients are added and then brought to a molten state. Frequently alloying constituents having a large particle size (i.e. greater than 0.15 mm. or 100 mesh) are used in order to avoid the aforementioned solubility problems as well as undue losses due to the more rapid oxidation of extremely fine material. When such relatively large particles, whether loose or in compacted form, are placed in a cold furnace with raw aluminum and heated, the first aluminum to become molten will surround the particles or penetrate into the compact. As this occurs, alloys of aluminum will be formed sooner than desired. These alloys have melting points much higher than the final operating temperature of the molten bath. The high melting aluminum alloys, containing significant amounts of manganese, chromium, or other alloying constituent, will then be lost from the final alloy. The composition of the final alloy is thus quite unpredictable.
The current practices for addition of alloying additives to aluminum thus require that all of the materials charged to the furnace must be heated to form a molten bath before any additive or hardener can be added. In order to simplify heat make-up operations, conserve fuel used in preparing the melt, and speed production of the alloy, it would be desirable to have an alloying additive or hardener which could be added to the other materials when charged to the melting furnace. This is known as "cold charging", or the addition of solid aluminum to an essentially empty melting furnace prior to actual melting. The most desirable additive is one which can be both cold charged and/or added to a molten bath. This permits maximum flexibility in alloy production.
Because of the disadvantages of the current state of the art, it remains desirable to provide an alloying additive and process for incorporating metals such as manganese, chromium, and iron into aluminum which will: (1) contain substantially only manganese, chromium, or iron so that smaller quantities are required for alloying; (2) be capable of dissolving in a molten bath of aluminum in short periods of time with high recoveries of the alloying constituents in the final alloy cold charged or added to a molten bath; (3) be adaptable to the technique of charging to an essentially empty furnace prior to melt-down of the charge ingredients; (4) not be limited in use to bath temperatures of 1,375.degree.F. (745.degree.C) or higher; (5) utilizes very fine particle size alloying constituents; and (6) not require the use of protective packaging or special handling procedures during storage and shipping.